The present invention relates to a digital optical transmitter for an optical fiber data link and, more particularly, to a driver circuit for driving a light emitting diode which serves as a light source in the optical transmitter.
Practical use of an optical fiber data link began with relatively low speeds on the order of 1 Mb/s and, today, it is extending to speeds as high as several hundreds of Mb/s to provide for a data link between large scale computers. So far as optical transmitters for low speed optical fiber data links are concerned, a light emitting diode (LED) is now predominant as a light source over a semiconductor laser due to the reliability of operation and the simplicity of a driver circuit. In parallel with such a trend, a remarkable development of high-speed response LEDs is under way and it is believed that a high speed optical fiber data link using such an LED as a light source will soon reach a practical stage.
Where a light source for an optical transmitter in an optical fiber data link is implemented by an LED and signal modulation at high speeds above several tens of Mb/s is desired, a driver circuit constructed as shown in FIG. 1 is extensively used (Stewart D. Personick, Optical Fiber Transmission Systems: Circuit For LED Transmitters, pages 47-53, Plenum Press, New York And London, 1981). As shown, the driver circuit includes input terminals 1 and 2 to which input signals having phases inverted relative to each other are applied. Transistors 3 and 4 constitute in combination a differential amplifier. An LED 6 is connected to the collector of transistors 3. A transistor 5 cooperates with resistors R12 and R14 to provide a current source circuit. The reference numeral 7 designates a power source input terminal 7 to which a positive DC voltage is applied. In this construction, the transistors 3 and 4 are alternately turned on and off by an input signal so that smooth charging and discharging occur for the junction capacity of the LED 6. Nevertheless, current consumption by such a driver circuit is undesirably great because either one of the transistors 3 and 4 is turned on with no regard to an input signal pattern and, thereby causes a predetermined current to constantly flow through the transistor 5.
One approach to cut down the power consumption in the above-described circuit is disclosed in Japanese Patent Laid-Open Publication No. 57-31666, published July 6, 1982. The approach consists in applying a clock signal having a predetermined period to the transistor of the current source circuit of the above-described circuit in order to drive a light emitting element only when the clock signal is high level. The problem encountered with the disclosed drive circuit is, however, that it requires the synchronization in advance between the input signal and the clock signal to properly trigger the switching transistor. In other words, it never permits the connection of an asynchronous signal to the input terminal. In addition, since the current source circuit is clocked at a predetermined period, a current flows therethrough even when an input signal is absent, resulting in wasteful current consumption.